In the sector of pre-press activity known as graphic and reprographic arts an intensive use is made of contact copying materials to produce screen dot images, line work and typesetting work. Both negative working photographic materials which produce negative-positive or positive-negative copies are used as well as so-called direct positive working materials giving rise to negative-negative or positive-positive reproductions.
In order to obtain exact copies with sharp dot and line edges, it is necessary to use fine-grained relatively insensitive photographic emulsions. The materials containing this type of emulsions are image-wise exposed in contact with the original in a graphic arts copying apparatus by means of high intensity radiation, preferably by light sources emitting a high content of near-ultraviolet light.
The handling of ever increasing amounts of photographic materials of different kinds, the decentralisation of the distinct steps in the reproduction cycle etc., have created a demand for silver halide materials which can be handled under clear ambient light illumination. This demand has given rise to the development of so called "roomlight materials" which can be image-wise exposed, handled and processed for a reasonable time while being illuminated by common office fluorescent tubes and daylight penetrating through office windows without the occurence of fogging of the negative emulsion or bleaching of the prefogged direct positive emulsion. The benefits of this include ease of working and inspection of the element during exposure and processing, and generally more pleasant working conditions for the operators. Prior art material which can be handled under roomlight conditions has been described in e.g. U.S. Pat. No. 2,219,667 and GB 1,330,044.
Silver halide emulsions contained in such roomlight materials should exhibit adequate sensitivity and other sensitometric characteristics for image-wise exposure while showing no photographic response under ambient light conditions. It is the task of the emulsion designer to establish the optimal compromise between these two conflicting characteristics.
Relatively sensitive direct positive emulsions can be composed of AgBr or AgBrI; in this case however red safety light conditions as present in classical darkrooms are required. Rather insensitive direct positive AgBr(I) emulsions which can be handled for short periods under bright light conditions can be obtained through the addition of a large amount of filter dyes although this often has a negative effect on the photographic sensitometric characteristics of the direct positive material such as a substantial decrease of the gradation. For roomlight direct positive applications it is therefor, like in the corresponding negative case, necessary to use emulsions with a high content of chloride so that there is a minimum overlap between the intrinsic sensitivity and the roomlight spectrum. Indeed, silver chloride emulsions show the advantage of a shorter spectral cut-off in the visible region.
However, for good reversal characteristics of a direct positive silver halide emulsion, particularly D.sub.min, silverbromide is more desirable than silver chloride containing emulsions. Furthermore, because of the greater solubility of silver chloride than silver bromide in commonly used rapid access developers it is more difficult to obtain good reversal characteristics with AgX emulsions containing high amounts of chloride than it is with emulsions with low contents of chloride.
Photosensitive materials with direct positive silver halide emulsions providing low D.sub.min and which do not suffer from an increase of D.sub.min upon long-range storage, and further satisfying two contradictory requirements, one being to provide low D.sub.min, and the other to have a satisfactory safelight aptitude, are very much desired in graphic arts.
Attempts to improve D.sub.min of direct positive silver halide emulsions have been described and are mostly based on an optimalization of the electron accepting system of the direct positive emulsions. Direct positive working emulsions can function according to internal and/or external electron trapping technology.
Internal electron trapping systems are described i.a. in "Chimie et Physique Photographique" by G.F. Glafkides, in "Photographic Emulsion Chemistry" by G.F.Duffin, in "Making and Coating Photographic Emulsion" by V.L.Zelikman et al, and in "Die Grundlagen der Photographischen Prozesse mit Silberhalogeniden" edited by H. Frieser and published by Akademische Verlagsgesellschaft (1968). As described in said literature internal electron trapping systems can be obtained through the presence of phase boundaries in the so.called core-shell emulsion type. A chemical sensitized core-shell type direct positive emulsion can be obtained through the chemical ripening of the AgX.core. as is commonly applied for negative silver halide emulsions, followed by a shell-type silver halide precipitation. Inorganic desensitizers, e.g. metal dopants such as Ir, Rh, Ru, Pb, Cd, Hg, Tl, Pd, Pt, or Au can function as well as internal electron traps in the interior of the silver halide crystals. In these cases the emulsion surface can be fogged or not. In the latter case usually a reducing agent is present in the photographic material or in its developing solution, e.g. tin compounds as described in GB-A 789,823, amines, hydrazine derivatives, formamidine-sulphinic acids and silane compounds. Chemical sensitization can be carried out by effecting the ripening if performed in the presence of small amounts of compounds containing sulphur e.g. thiosulphate, thiocyanate, thioureas, sulphites, mercapto compounds and rhodamines or through the addition of gold-sulphur ripeners.
According to the principles of external electron trapping the emulsion surface is prefogged and an electron-accepting compound is adsorbed at it. Prior art concerning electron-accepting compounds suitable for use in direct positive emulsions, includes nitrostyryl and nitrobenzylidene dyes as described in U.S. Pat. No. 3,615,610, dihydropyrimidine compounds of the type disclosed in DE 2,237,036 and compounds of the type disclosed in U.S. Pat. No. 3,531,290. Other useful electron accepting compounds are cyanine and merocyanine dyes containing at least one nucleus, and preferably two nuclei with desensitizing substituents such as nitro groups, or dyes containing desensitizing basic nuclei as described in U.S. Pat. Nos. 2,930,644, 3,431,111, 3,492,123, 3,501,310, 3,501,311, 3,574,629, 3,579,345, 3,598,595, 3,592,653, and GB 1,192,384.
If however the absorption spectrum of an external electron-accepting agent would extend to the visible region, as it is e.g. the case with the well known electron-accepting agent Pinakryptol Yellow, this would lead under roomlight conditions to the bleaching of the developable specks created by the prefogging of the emulsion. Electron accepting compounds suitable for use in roomlight insensitive emulsions are disclosed respectively in US 4,820,625 and EU application number 90200646,9. These electron accepting compounds belong to the class of nitro-substituted phenyl thioether derivatives and nitro-substituted heterocyclic thioether derivatives, e.g. pyridine derivatives. In the later applications concerned with an insensitive direct positive roomlight material, the emulsion layer(s) preferably additionally contain(s) a ntroindazole or nitrobenzimidazole derivative, preferably a 5- or 6-nitroindazole or 5- or 6-nitrobenzimidazole. These organic compounds have proved to be very useful in further reducing the minimal density level if combined with the prescribed electron accepting compounds and to provide good storage properties.
It is an object of the present invention to provide a direct positive silver halide photographic material containing a prefogged direct positive silverchlorobromide emulsion, which can be handled in bright safelight conditions and which shows good reversal photographic characteristics, in particular, an adequately high maximim density and low minimum density, high contrast, no re-reversal effects and a stable sensitivity even after long-range storage.
Other objects will become apparent from the description hereinafter.